I suspect that everyone will find their own personal level, depending on the type of insulin, its profile and how they respond to it. I was advised to have a snack if much below 8 at bedtime, but I can't lie down to sleep if I've just eaten - so I don't actually eat unless I'm around 6 these days. I've never eaten supper, so don't want to start now.
What I tend to do is check mid-evening - usually my post-dinner reading - and if I feel hungry, or we ate earlier than usual, might have some nuts, yogurt or cheese at that time. Chances are that if I'm likely to need a little booster, it'll be about the time my husband announces that he has the munchies and heads for the cornflakes.
I don't routinely have carbs unless I'm really going too low - which I did for a while in January before a change in insulin.
I'm happiest going to bed between upper 6s to mid 7s - I feel as though I stay pretty even overnight at that.